Antonio Cianfrani. Dynamic Host Configuration Protocol (DHCP)

Transcription

1 Antonio Cianfrani Dynamic Host Configuration Protocol (DHCP)

2 DHCP protocol The DHCP protocol allows to dynamically configure hosts in a LAN. Configuration parameters: IP address and subnet mask Default gateway DNS Server

3 Example Server DHCP Client DHCP /24

4 Server DHCP configuration (1/3) The command to start DHCP configuration is Router(config)#ip dhcp pool NAME_POOL NAME_POOL is the name used to identify the DHCP configuration on the router The next step is the definition of the address pool to be assigned dynamically: Router(config-dhcp)# network NET_ADDRESS NETMASK In the case of /8 block: Router(config-dhcp)# network

5 Server DHCP configuration (2/3) It could happen that the set of available IP addresses to be assigned dynamically is not exactly the pool Motivation: a host (usually a server) requires a static (fixed) IP address It is possible to exclude IP address from the pool Router(config)#ip dhcp excluded-address IPADDRESS(ES) If the initial 49 addresses should be excluded: Router(config)#ip dhcp excluded-address

6 Server DHCP configuration (3/3) To provide the Internet access, default router ( ) and DNS server ( ) must be configured Router(config-dhcp)# default-router Router(config-dhcp)# dns-server

7 Antonio Cianfrani Network Address Translation (NAT)

8 Public and private IP addresses Problem: IP addresses are not enough to assign a unique IP address to each network host/device

9 NAT (1/4) Advantages No need to request public IP addresses for all the devices Reduction of public IP addresses Security improvement: real devices IP addresses hidden Drawbacks: Extra-load for access routers Breaking of the layers separation rule

10 NAT (2/4) Inside Network Outside Network Gateway Internet

11 NAT (3/4) The IP addresses of the local network ( X) are not visible from outside. The router (Gateway) has two network interfaces, one on the local (inside) network and one on the public (outside) network. The router is connected to the Internet Service Provider (ISP), that will assign the public IP address to the outside interface

12 NAT (4/4) The router must implement NAT (Network Address Translator - RFC 1631) The NAT idea id to modify (translate) the IP addresses (and other header fields) of outgoing and incoming packets. Two main types of NAT operations exists: Static NAT: one-to-one translation; Dynamic NAT: N-to-one translation.

13 Static NAT Internal Network Outside Network SA NAT SA Internet/Intranet SA = Source Address Inside Local IP Address NAT Table Inside Global IP Address The translation is performed also for the incoming packets (acting on the IP destination address field)

14 Dynamic NAT Inside Network Outside Network SA NAT SA Internet SA = Source Address Tabella NAT Inside Local IP Address & port Inside Global IP Address & port :80 SERVER HTTP : : : :55002

15 NAT Configuration The first step in NAT configuration is the setting of inside (local) and outside (global) interfaces Router(config-if)#ip nat inside Router(config-if)#ip nat outside

16 Static NAT configuration One-to-one translation among a private and a public IP address. Used for servers. Translation rule: Router(config)# ip nat inside source static private_address public_address

17 Dynamic NAT configuration (1/2) The number of available public IP addresses is lower than the number of hosts (private IP addresses) requiring Internet access Private IP addresses definition: Router(config)# access-list access-list-number permit source_address wildcard Pool of public IP addresses: Router(config)# ip nat pool name start-ip end-ip netmask netmask Translation rule Router(config)# ip nat inside source list aclnumber pool name

18 Dynamic NAT configuration (2/2) It is also possible to use a single public IP address for all the LAN hosts: the public IP address of the router In this case the pool of public IP addresses is not required Translation rule: Router(config)# ip nat inside source list aclnumber interface interface overload